Mostly working ZX80

Discussions about Sinclair ZX80 and ZX81 Hardware
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gregc
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Mostly working ZX80

Post by gregc » Thu Aug 09, 2018 1:23 pm

Hi, I am new to this forum and hope someone can help. I used to have a large computer collection but it has to be reduced and it is now the early Sinclairs and the Sharp MZ stuff. I have a ZX80 and ZX81 as well as a ZX80.5 - in other words an 80 upgraded to 81 ROM and keyboard overlay.

All my computers work except my ZX80 has a problem. If I tap onto the input to the RD modulator then I can see a lovely picture on my Sony monitor. There seems to be nothing at the RF output though. That sounds like the modulator has failed but is it possible to test and does anyone know of a place I can buy a replacement which will work? (bought one off eBay which was no better for a different computer). I have access to test equipment including an oscilloscope so can sort out any testing.

I bought the ZX80 off eBay a long time ago and I am not sure if it had been converted to composite out but plugging the output to my monitor does not work. Is there any indication inside the RF unit that it has been modified?

Any other ideas very gratefully received!!

Greg

RWAP
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Re: Mostly working ZX80

Post by RWAP » Thu Aug 09, 2018 1:37 pm

It is probably more a case that the TV cannot pick up the signal from the ZX80 (as it lacks a back porch signal).

The options are to fit a ZXVid or ZX8-CCB - you can find both options on www.sellmyretro.com :D

However, I would recommend that you wait for advise to actually ensure that the ZX80 is working first!

gregc
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Re: Mostly working ZX80

Post by gregc » Thu Aug 09, 2018 4:22 pm

Hi,

I purchased a Philips 7300 TV Tuner a while ago which has sorted these issues. It has proper analogue tuners (rotating dials!) and then outputs to a modern TV via a composite signal. It has 12 channels which is enough for all my computers. It is only the ZX80 that fails. It must be transmitting something to get a signal on the monitor but might be the wrong amplitude perhaps.

Shall look into your suggestions.

Greg

gregc
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Re: Mostly working ZX80

Post by gregc » Thu Aug 09, 2018 4:34 pm

Hi Rich,

Both items are really clever. Have to say I prefer the latter (ZX8-CCB) as it leaves the RF modulator as sold. Might be a good option even if I get the RF working.

Greg

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1024MAK
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Re: Mostly working ZX80

Post by 1024MAK » Thu Aug 09, 2018 4:54 pm

Hi Greg

The modulator is simple in principle, but a bit more complex in practice. It contains a UHF oscillator. The composite video signal causes the UHF oscillator output to change amplitude slightly (amplitude modulation). The result being the composite video is modulated onto the carrier signal (the carrier being the UHF oscillator frequency).

The modulator requires just a +5V DC supply, ground (via it's case) and a composite video input signal. Even without a composite video signal, your analogue tuner and TV should be able to show the difference between a UHF carrier and the random noise of a "empty" channel (or at least, empty of a analogue TV transmission).

You can take the lid off, if you do (using a screwdriver), it should be obvious if a composite modification has been done, as if so, there will be a wire or capacitor, or transistor connected between the composite video input wire/pin and the centre tab of the output "phono" connector. If however the wires go to PCB that also has coils in it, then its original.

In theory it should be possible to repair a modulator, but in practice, I've never heard of anyone trying. As you really need a very high specification 'scope or some very good luck and lots of time to tinker...

Mark

gregc
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Re: Mostly working ZX80

Post by gregc » Thu Aug 09, 2018 9:44 pm

Hi Mark, thanks for that. Have had a quick look and the RF modulator is standard. I shall have another look on the scope and on the TV tomorrow to see what I can find. I might still purchase the ZX8-CCB anyway as it will show a much sharper screen but leave the modulator as original.

Greg

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